Abstract

While the vast majority of metastatic carcinomas to the skin occur late in the disease course as a reflection of widespread systemic involvement and carry poor prognosis, some cases present as the first sign of disease. We present a case study of a 63-year-old male, long-term smoker with a medical history of chronic obstructive pulmonary disease, and recent pneumonia. Additionally, the patient was diagnosed with metastatic rectal carcinoma at stage IV, involving the liver and mediastinum. The patient presented with multiple skin nodules of varying sizes, located on his scalp, neck, chest, abdomen, and back, that were suspected clinically to represent metastatic colon cancer. Microscopic examination revealed nests of high-grade, squamoid-looking cells infiltrating the dermis with extensive vascular invasion indicating a high-grade carcinoma with squamoid differentiation. Immunohistochemistry excluded colon adenocarcinoma as the primary tumor and confirmed the metastatic cells as squamous, supported by positive staining for p40, CK5/6, and AE1/3, and negative staining for ck20, and cdx. Subsequent imaging detected a 3.2 cm pulmonary nodule in the left lower lobe. Detailed histopathological and immunohistochemical evaluations confirmed the presence of a distinct second primary tumor, specifically squamous cell carcinoma originating from the lung. This case highlights the complexity of concurrent primary malignancies and emphasizes the need for a multidisciplinary approach in accurate diagnosis. This case report aims to contribute to the existing literature on the clinical presentation, diagnostic challenges of concurrent primary malignancies, particularly in the context of SCC of the lung arising in patients with a history of colon adenocarcinoma.